The plasma arc cutting process, during the two decades or so since its development, has become a firmly established production tool in the metal fabrication industry. Its wide acceptance is the result of unique advantages such as the ability to cut any metal at relatively high cutting speeds. These advantages are made possible by features that are characteristic to this process only: namely the extremely high temperatures and high gas velocities which are needed to turn the more common electric arc into a plasma arc. Essentially, a plasma arc differs from the common electric arc in the following way. Whereas the electric arc cutting (or welding) process requires the electrical current to jump across an intervening air space between the cutting or welding head and the metal workpiece, the plasma arc process provides a high-velocity stream or jet of gas which is directed toward the workpiece, and which literally carries the current with it by virtue of being highly ionized. In effect, the gas is forced to pass first in front of an electrode contained in the cutting torch head, from where it passes in a high-velocity jet directly to and against the metal workpiece.
Despite the versatility of the plasma arc cutting process, and the high cutting speeds which it makes possible, the same characteristics which lead to these advantages also are associated with important drawbacks. For one, the high temperature utilized is responsible for the generation of toxic fumes such as ozone and oxides of nitrogen, and also produces fine metal particulate matter in the cutting fumes. The supersonic gas velocity, needed to blow out the molten metal in order to obtain high cutting speeds, is responsible for the high noise levels generated by the process. Finally, intense ultraviolet radiation is emitted by the arc. These characteristics, even though they have always been inseparable parts of the plasma arc cutting process, are receiving more scrutiny in this era of increased health and safety consciousness. The health hazards arising from these characteristics are presently the only real obstacle to an even wider acceptance of the plasma arc cutting process.
The conventional way of coping with the problems just mentioned was to employ a so-called water table under the metal plate being cut, and to make use of water mufflers. The principle here is that the smoke passing through and under the plate being cut was scrubbed by the water in a boxed-in table, while the noise level and the ultraviolet radiation were reduced by a water "curtain" generated around the lower end of the torch by a water muffler. These methods work well enough if there is no gap between the bottom of the plate and the underlying water table, but since the plates are seldom straight, gaps do not occur and these allow fumes to escape from under the plate. Any such gaps also tend to act as echo chambers, and accentuate the noise generated by the process.